The present invention relates to a polycrystalline silicon thin film (hereafter simply called the poly-Si film) and a semiconductor device wherein the poly-Si film is applied.
A poly-Si film which is grown according to a conventional growth method, for example, grown by a low-pressure CVD (Chemical Vapor Deposition) system at a temperature of 700.degree. C. with a silane gas of a flow rate of 800 sccm, has a layer structure wherein percentage of grains of a preferred orientation (110) increases monotonously towards the film surface, as can be seen in FIG. 8.
FIG. 8 is a cross section schematically illustrating a poly-Si film 83 grown on a SiO.sub.2 surface oxidation layer 12 of a Si substrate 11 according to the conventional method. In FIG. 8, a numeral 831 denotes a base layer where the preferred orientation rate (110) is about 30%, a numeral 833 denotes a surface layer where the preferred orientation rate (110) is about 70%, and a numeral 832 denotes an intermediate layer where the preferred orientation rate (110) varies from about 30% to 70%. In the drawing, the base layer 831, the intermediate layer 832 and the surface layer 833 are bounded by dashed lines schematically. However, there is no distinct boundary between them actually, and the preferred orientation rate varies continuously traversing the layers, which is the same in other drawings when there is no other description.
A poly-Si film of another preferred orientation such as (111) or (100) obtained by the conventional growth method shows a similar layer structure, that is, a structure wherein the preferred orientation rate increases monotonously towards the film surface.
However, problem of excessive dopant diffusion into base material or insufficient activation of the poly-Si film itself becomes more serious with the poly-Si film having such layer structure, as it is made thinner and thinner along with miniaturization and high-performance of the semiconductor device whereto the poly-Si film is applied.
FIG. 9 is a cross section schematically illustrating a MOS (Metal Oxide Semiconductor) transistor, wherein a source electrode 95, a drain electrode 96, field oxidation films 93, a gate oxidation film 94, a gate electrode 97, Al (aluminum) wirings 98 and a passivation film 99 are configured on a n-type Si substrate 92.
When the poly-Si film having such layer structure is used for the gate electrode 97 without sufficient thickness, the dopant diffuses into the n-type Si substrate 92 penetrating the gate oxidation film 94, which causes a considerable variation of on/off characteristic of the MOS transistor and an operational delay because of unnecessary gate capacitance, both disabling to improve operational speed of the MOS transistor.
FIG. 10 is a cross section schematically illustrating a thin film transistor to be applied for driving an LCD (Liquid Crystal Display), for example, wherein an active layer 101 comprising a source electrode 102 and a drain electrode 103 is configured on a glass substrate 100, together with a gate electrode 104, Al wirings 105, a gate oxidation film 106 and a passivation film 107.
When the above conventional poly-Si is applied to the active layer 101, operational speed of the thin film transistor of FIG. 10 is also degraded because of small carrier mobility of the poly-Si film. Therefore, it can be said that the thin film transistor to be used for high-speed driver of the high quality LCD cannot be realized with the poly-Si film obtained according to the conventional growth method.
As above described, in the film transistor using the poly-Si film as its active layer, or in the MOS transistor using the poly-Si film as its gate electrode, characteristics of the poly-Si film, such as the diffusion and precipitation of dopant, the interface and surface state or the carrier mobility, which define characteristics of the transistor, becomes difficult to be controlled, when thickness of the poly-Si film is to be reduced according to reduction of the element size of the MOS transistor or the thin film transistor. In other words, performance improvement of the LCD or the IC is now being limited by electronic characteristics of the poly-Si film applied to the film transistor or the MOS transistor used therein.
Therefore, a primary object of the present invention is to provide a poly-Si film which has excellence in its characteristics themselves concerning transistor characteristics, such as the diffusion and precipitation of dopant, the interface and surface state or the carrier mobility, and excellence in controllability of those characteristics as well.
Another object of the invention is to provide high-quality semiconductor devices by applying this poly-Si film.